1. Field of the Invention
The present invention relates to kilns for drying lumber. More particularly, the present invention relates to a vacuum chamber kiln.
2. Description of the Related Art
The use of lumber drying kilns is widespread for preparing lumber for use in building furniture, flooring, and other applications where warping of lumber during and after incorporation in the product or structure is not acceptable. In the past, lumber was air dried or cured in stacks, either outdoors in the weather, or indoors, taking lengthy time to reach the required dried or cured conditions. Drying kilns have speeded the drying or curing of wood to a relatively short time and produce more consistent quality lumber.
There are differing ways to kiln dry lumber. Steaming and dehumidification are examples. The drawbacks to these are the time and energy it takes to achieve the desired final moisture content in the wood. It typically takes from several weeks to months to dry lumber in these types of kilns, depending on the type of wood and the thickness of the lumber to be dried.
A relatively recent development in lumber drying kilns is the vacuum kiln. A vacuum kiln can reduce drying time from weeks to days and from months to weeks, depending on the thickness of the lumber to be dried. Present vacuum kilns have, however, various drawbacks. Vacuum kilns cost as much as twenty times that of conventional kilns and, typically, they are limited in size, having a capacity of 30,000 board feet or less of lumber.
In dehumidification kilns, layers of lumber are stacked on stickers (lumber separators which allow circulation of air or steam) to form a stack which is within the capacity of the kiln. The temperature inside the kiln is raised to between 110-185 degrees F. A large dehumidifier, similar to those used in homes, constantly removes excess moisture in the air while large fans circulate this dried air around the wood in the kiln which is then recycled to the dehumidifier. As the dry air passes around the wood, water molecules from the outer surface of the wood changes from liquid to water vapor and this moisture is removed in the dehumidifier where liquid water is removed from the system. Although outside fresh heated and dehumidified air could be supplied to the wood-containing compartment of the kiln and the resulting moisture-bearing air released into the atmosphere, this is relatively energy inefficient. In most kilns, the heated air is circulated between the wood-containing compartment and the dehumidifier, thus retaining heat within the system and requiring only the addition of heat in amounts necessary to make up for heat losses through radiation and removal of hot water. Because of the moisture gradient formed in the lumber during drying, water begins to travel from the inside towards the outer surface of the lumber being treated. This process is continued until the desired moisture content in the lumber is obtained, at which time the lumber is removed from the kiln for use.
In a vacuum kiln, layers of lumber are either stacked on stickers as in the dehumidification kiln, or on hot plates separating the layers of wood until the desired stack is obtained. The hot plates are typically large, flat hollow structures through which hot water is circulated by means of a hot water supply and conduits to and between the hot plates. Temperatures inside these kilns are similar to those reached in conventional dehumidification kilns. An airtight chamber capable of handling vacuums of up to 29.9 inches of mercury is employed to house the lumber during the drying process. These, chambers must be of high strength to withstand the atmospheric pressure without collapse. Also, the chamber must be constructed of an inert material such as stainless steel, due to the corrosive nature of the acids which are removed from the wood during the drying process. The main cost and size-limiting factor in vacuum kiln construction is the stainless steel chamber.
After the stack of lumber has been placed inside the kiln chamber and the door sealed, the drying process may begin. A vacuum is pulled on the lumber by means of a vacuum pump connected with the interior of the kiln chamber and exhausting to the outside. As the vacuum increases, the moisture in the lumber is boiled out of the lumber at temperatures below the boiling point of water (if the vacuum is sufficiently high, the water will boil at room temperature). The steam or water vapor released by the lumber inside the chamber is passed through a condenser and then pumped to the outside of the chamber. As the moisture inside the lumber boils and is released, the temperature of the lumber drops. This is due to the fact that latent energy in the moisture within the wood turns to steam and leaves the wood. To compensate for this loss in energy, heat, must be added to the chamber to prevent freezing of the wood or the slowing of the drying process. Since heat does not travel well through a vacuum, direct heating by contact of the layers of lumber is accomplished through the intervening heating plates. As mentioned above, these plates are typically hollow and allow heated water to pass through, typically in series by connecting conduits at the end or side of the lumber stack. These direct contact kilns are the fastest available but do take considerable effort to load and unload due to the effort involved in assembling and disassembling the lumber stack with the intervening hot plates.
Other vacuum kilns are operated with lumber layers separated by stickers in the conventional stacking technique and circulate a certain amount of hot air inside the chamber. They are typically slower than hot plate systems but are much quicker to load and unload.
Present vacuum kilns are, then, unpopular for two reasons. First, the initial cost of the kiln is prohibitive for many kiln operators. Second, the relatively small capacity of present vacuum kilns make them undesirable for many other kiln operators.
It would be desirable to provide a vacuum kiln design for drying or curing wood lumber which may be built for substantially less cost than present vacuum kilns. It would also be desirable if such a kiln design would be readily scalable to make small units affordable to a hobbyist as well large capacity units satisfactory for large lumber kiln operations.